Heat the oven to 200C/fan 180C/gas mark 6 (convection fan oven at 350F).
Put 1kg potatoes, peeled and halved, in a saucepan and pour over enough water to cover them. Bring to the boil and then simmer until tender.
When cooked, drain thoroughly and mash with a splash of milk and a knob of butter. Season with ground black pepper.
Put 25g butter, 25g plain flour and 4 finely sliced spring onions in another pan and heat gently until the butter has melted, stirring regularly. Cook for 1-2 mins.
Gradually whisk in 400ml milk using a balloon whisk if you have one. Bring to the boil, stirring to avoid any lumps and sticking at the bottom of the pan. Cook for 3-4 mins until thickened.
Take off the heat and stir in 320g-400g mixed fish, 1 tsp Dijon or English mustard, a small bunch of finely snipped chives, handful of sweetcorn and handful of petits pois. Spoon into an ovenproof dish or 6-8 ramekins.
Spoon the potato on top and sprinkle with a handful of grated cheddar cheese.
Pop in the oven for 20-25 mins or until golden and bubbling at the edges. Alternatively, cover and freeze the pie or mini pies for another time.
Video
Notes
Just to let you know if you make this make dish, make sure you melt butter first then add the flour keep sturing it for about 2 mins use a timer. The flour has to cook so you don't get a horrible flour gritty texture sauce.